The present invention relates to conveyor systems and, more particularly, to conveyor systems which must convey selected items placed thereon from a region of one temperature to a region of a significantly different temperature while supporting such items at edges thereof.
Conveyors are used for many purposes. Among them is the transporting of items to be heated from an unheated access region into a heating region. In those situations where there is a significant temperature difference between the heating region and the access region, the conveyor is subject to substantial thermal gradients. Such thermal gradients can lead to substantial stresses on the conveyor structural components. The conveyor structural components must be capable of operating in these conditions.
One kind of conveyor encountering these conditions is a conveyor for supporting the edges of printed circuit boards that are to be conveyed from an access zone into a heating zone for the purpose of causing affixing thereto of electronic components thereon. The supporting of these printed circuit boards at edges thereof only permits both sides of the printed circuit board to be heated simultaneously to cause such fixation, or heated alternatively for this purpose in subsequent passes through the heating zone. This capability is desirable in the fabrication of many kinds of printed circuit board systems.
A conveyor for accomplishing this can be provided by use of a pair of chains which, in one manner or another, support edges of circuit boards between them. The chains can then be driven by an actuation means connected to sprockets around which the chains are arrayed. The metallic nature of the chains provides a conveying means which can withstand substantial stresses.
Such chains must each be guided by some kind of a guiding arrangement to permit the chains to move while causing them to stay a selected distance from one another if the printed circuit boards are to be continuously supported at their edges without coming off either chain. Since only a very small portion of the edges of a circuit board will be over each chain, a rather small tolerance in the spacing of one chain from the other is permitted. Thus, the guide rails must set a rather accurate path along which the chains can move.
Choosing a guide rail construction to meet this requirement usually comes down to two choices. One is a guide rail having substantial cross-sectional dimensions so that it is quite rigid but with relatively little support on the conveyor frame, perhaps just at each end. The alternative is providing frequent supports on the conveyor frame if the guide rail is to have smaller cross-sectional dimensions.
A relatively larger but rigid rail is definitely more costly to use than is a rail of a smaller dimensional construction. A further difficulty with a relatively large rigid rail is that heating will cause such a rail to expand, and this expansion can lead to the rail bending between its support points on the frame with the result that the rail has a bow in it so that it has a portion with a lateral displacement away from its desired location. Such a displacement will carry the chain with it leading to the possibility of the edge-supported printed circuit boards being carried thereby falling from the chain.
The use of more supports generally, whether with such a relatively large, rigid guide rail or one of less rigid construction, leads to greater heat losses because of heat transfer through such supports to the conveyor frame. Further, with the rail situated inside a heating region, adjustment of the rail to its desired location can be quite difficult because of the lack of good access to the rail and such supports.
Thus, there is desired, for a conveyor extending into a heating zone using a guide rail, an arrangement which will stiffen the guide rail against lateral displacement of portions theeof due to forces tending to cause such displacement without having to use a relatively large cross section rigid rail, and without having to use a large number of rail supports. In addition, the arrangement should allow a rail to be selectively moved laterally with respect to the direction of travel of the chain to permit adjustment for carrying selected items of varying widths.